Experimental Research And Numerical Simulation On The Woven CFRP Short Pulsed Laser Milling

Woven Carbon Fiber Reinforced Plastics(CFRP)short pulsed laser milling is a non-contact,highly flexible material removal technology with small heat affected zone which has a good application prospect in the field of aerospace.Through the layer by layer laser ablation,holes,pockets and slits in CFRP could be successfully manufactured.With extremely high energy density,the interaction of short pulsed laser with CFRP is a coupling process including heat transfer,phase change and mass transfer and the material removal mechanism is much complex.Meanwhile,the laser thermal effect causes various defects,such as resin matrix pyrolysis,fiber exposition,which reduce the mechanical properties of the materials.In addition,as the energy of a single laser pulse is small,the material removal in macro-scale size should be realized by the laser milling process in which the machining area is hatched by scanning laser lines and material is removed layer by layer.Selection of the processing parameters has a significant impact on the material removal process.Problems including the material removal mechanism,heat damage,process planning,etc.in the CFRP short pulsed laser milling process are studied in this paper.The combination of theoretical analysis,experiment design and numerical simulation is carried out.The main research contents and related conclusions are as follows:(1)Experimental study on the woven CFRP short pulsed laser millingThe experiments of woven CFRP short pulsed laser milling were carried out to study the laser milling characteristics and the effects of machining parameters.Through the single factor screening experiment,the mapping relationship between the parameters such as average power,hatching distance,scanning speed and the machining defects such as heat affected zone and rough machining surface was analyzed.In order to analyze the interaction of processing parameters,the response surface desigh of experiment was carried out,and a polynomial fitting model with response to the material removal rate was obtained.The factors influencing the material removal were obtained by the variance analysis.Through microscopic observation of the machined surface and product analysis,the microscopic ablation of the short pulsed laser and the mechanical erosion process of the material were confirmed experimentally.A milling process planning method based on the control of pulse overlapping rate was proposed to maximize the material removal rate.(2)Analysis of the laser absorption characteristics of the woven CFRPBased on the theory of laser electromagnetic wave propagation and the optical properties of carbon fiber uniaxial crystals,the reflection and refraction behaviors of light at the interface of carbon fiber/resin were studied.The absorptivity model of a single fiber was established.By studying the propagation modes of a ray in the microstructure of CFRP,the uniformly distributed carbon fiber ray tracing model was established.The effects of microstructure and laser parameters(polarization,energy distribution)on the laser absorptivity were obtained,revealing the selective absorption properties of CFRP.By means of the improved carbon fiber random placement method,a randomly distributed carbon fiber material model with a carbon fiber volume fraction of up to 60% was generated,and the corresponding ray tracing model was established to carry out the absorption analysis of the real material.The laser absorption measurement of CFRP wes carried out to verify the model.(3)Response behavior of the woven CFRP under short pulsed laser irradiationConsidering the high heating rate and cooling rate characteristics of the interaction between the short pulsed laser and the material,the pyrolysis mechanism of the resin was studied.A one dimensional numerical model including the surface ablation and the resin pyrolysis was established.The material ablation was simulated by the birth-death element method.The processes of heat transport and pyrolysis gas transport were numerically solved by sequential coupling calculation framework.The material response characteristics of CFRP short pulsed laser ablation were obtained.The effects of laser pulse width on the ablation and heat affected zone were studied.Variation of the flow pressure of the pyrolysis gas with the laser irradiation and the pulse width was analyzed,and the mechanical ablation effect was revealed.The study shows that the short laser pulse is beneficial to the removal of material and the inhibition of laser thermal effect,laying a theoretical basis for the application of short pulsed laser in CFRP processing.(4)Three dimensional simulation of the woven CFRP short pulsed laser millingBased on the characteristics of short pulsed laser milling,a three dimensional numerical model of woven CFRP laser milling was established.The material removal process and the dynamic formation process of heat affected zone were analyzed.Through the analysis of a single pulse laser ablation,the ablation morphology and temperature field characteristics of the material were obtained.The interaction between multiple pulses and the generation process of the inter-pulse island-shaped material region was analyzed.The accumulation of the diffused heat in the island-shaped area was accurately predicted.It was clarified that the mechanical erosion plays an important role on improving the material removal efficiency.Based on the conclusions above,the key process parameters including average power and spot spacing were analyzed,and the optimal process parameters for the machining with high efficiency and good quality were obtained.The experiments of short pulsed laser milling were performed to verify the model.(5)Experimental study on the application of the woven CFRP short pulsed laser millingBased on the woven CFRP short pulsed laser milling,the pocket milling and contour milling experiments were carried out.The material removal stability of a single layer and the interaction between the multiple layers were analyzed.Main characteristics of the cutting kerf were analyzed in the contour milling.The range and shape of heat affected zone were observed.The mechanical properties of the milled samples were tested.The influence of laser processing on the attenuation of the CFRP mechanical properties was clarified by comparison with the mechanical processed samples.